I have a 300 gallon Rubbermaid stocktank in the ground. I used to keep a floating heater in it all winter, but realized that it was keeping it all too warm, and unnecessary. Last year I just kept the pump going (over a small waterfall) and it seemed to keep the frogs alive longer. So that's what I continued to do this year. Except I pulled the 1" pipe out further over the pond. I thought it might oxygenate better. Well, I guess I hadn't looked out for a day or so, and this morning I realized that everything was frozen......including all the water from the end of the tube down to the pond. But the strange thing was, There were 2 levels of frozen ice........The first one on top and then space, and then another layer about 10" below it. The top layer had a big hole in it, and I could see the lower layer, which didn't have a hole. The top ice had expanded about 5" higher than where the water surface had been, and the second layer is lower. I hope the pump didn't get ruined. Anyhow......I got out a smaller deicer, to just make a hole in the lower layer, which appears is working. Any explanation why there are now 2 different layers of ice, about 5" apart?

Oh.....better mention that our temps for the past 2 days have been single digits and last night it was below zero.
I can't make sense of how this happened. Why didn't it just freeze in one layer on the water's surface?

I hope the frogs are okay (no fish).

Thanks!